The proposed experiments focus on differences in cell and tissue regulation between young and senescent mouse mammary epithelium. Using plastic implants capable of sustained, local, in vivo release of bioactive molecules, we have demonstrated that locally elevated cyclic adenosine monophosphate (cAMP) levels will reinitiate DNA synthesis, cell division and normal morphogenesis in aged gland which has depleted its normal potential for growth. These results strongly suggest that the senescent phenotype is due to a regulatory dysfunction(s) rather than general cellular deterioration. Our central hypothesis is that prolonged exposure to natural mitogens produces specific lesions that result in an irreversible decline in the cell's ability to respond to these mitogens. Using a combination of in vivo and in vitro methods, the following questions will be investigated: 1. What are the limits and characteristics of the cholera toxin-stimulated, cAMP response? Will senescent tissue grow so long as cholera toxin is present? The extended time-course of cAMP stimulation, as well as the dose-response will be studied. 2. What are the natural, cAMP-active mitogens possibly implicated in mammary aging? Several lines of preliminary evidence are presented strongly implicating arachidonic acid metabolites as regulators of mammary growth. The role of these agents in both aging and the growth of young gland will be investigated, and growth effects will be correlated with levels of cAMP and adenylate cyclase. 3. What is the role of epidermal growth factor (EGF) in mammary aging? EGF is mitogenic for mammary cells in culture, and our preliminary results indicate a growth-regulatory role in vivo. Using both cultured cells and implant techniques, we will study the role of EGF in growth and morphogenesis of young gland, and investigate possible loss of mitogenic effectiveness in senescent cells.